Basically because that's what your contract agreed to. I have something similar. I have 3 sick days per year and if I don't use them they become vacation PTO for next year. Regular pto doesn't roll over but sick days do.
That said, I'm not a fan of the combined system. In a lot of other ways, we expect employers to accommodate employees who may have issues of various kinds. Accommodating employees who get sick more often seems a reasonable extension of that. (And I say that as someone who, touch wood, has largely benefited from a combined system over the past decade or so.)
Because the alternative to being in bed with the flu is being at work with the flu, and the employer doesn't want to catch the flu? It kinda makes utilitarian sense.
As I said, I'm not a fan of the combined vacation and sick time in any case (in part for the reason you say). And, yes, people will drag themselves into work because they don't want to lose a vacation day.
If you hoard your PTO in case you get sick, you don't get to enjoy vacations.
There's a middle ground, but the spot where you draw the line and say "I need to save this many days in case I get sick" is going to be different for everyone, and most people are not going to calculate it correctly.
Not to mention the employee isn't incentivized properly here. The employee that takes too much vacation and goes to work sick, they're not the one who pays the cost of them going to work sick, it's the other employees who do. So a combined PTO scheme like this actually incentivizes employees to err on the side of taking more of it as vacation than they should.